1. Field of the Invention
The present invention relates to an ignition system for an internal combustion engine, and particularly, to an ignition system having at least two spark plugs for each of the cylinders, in which the number of spark plugs ignited is controlled in accordance with an operational condition of the engine.
2. Description of the Prior Art
There are conventionally known ignition systems having two spark plugs provided for each of cylinders in an internal combustion engine, wherein the switching-over of a two-point ignition in which both of the two spark plugs are ignited and a one-point ignition in which only one of the two spark plugs is ignited, is controlled in accordance with the magnitude of the load on the internal combustion engine, thereby providing a reduction in the amount of nitrous oxides (NOx) in the exhaust gas (see Japanese Patent Application Laid-open Nos. 63531/77 and 124676/81).
In a condition in which the temperature of the internal combustion engine is low, the air-fuel ratio is varied due to an increase in the amount of fuel adhering to the intake pipe and/or an insufficient atomization of the fuel, thereby resulting in an unstable firing performance provided by the spark plugs.
In an internal combustion engine including an intake valve opening and closing control mechanism for controlling the opening and closing timing of an intake valve to control the amount of air drawn in accordance with the demand load, the period of opening of the intake valve is shortened remarkably during an extremely low load operation, such as during idling, and after closing of the intake valve, a longer adiabatic expansion period is started and followed by a compression stroke. For this reason, the temperature during the compression is not increased sufficiently, resulting in an unstable firing performance provided by the spark plugs.